From the Supplement, compiled after Aquinas’s death from his Commentary on the Sentences and transcribed here from the 1917 to 1922 printings. About the Supplement.
Text: Summa Theologiae, Supplement to the Third Part (Supplementum Tertiae Partis), transcribed from Internet Archive scans, translated by the Fathers of the English Dominican Province. The Supplement, compiled after Aquinas’s death from his earlier Commentary on the Sentences, in the same English Dominican translation as the Gutenberg Parts. Transcribed by Bibliotheca Catholica from the OCR of four Internet Archive scans of the pre-1931 printings (Questions 1 to 33 from the 1917 Washbourne and Benziger volume, 34 to 68 from the 1922 volume, 69 to 86 from the 1921 volume, and 87 to 99 from the 1922 Benziger volume), with OCR errors corrected and doubtful passages marked, not rewritten. 99 questions and 446 articles. The two appendices printed after Question 99 are not included. Sources, SHA-256 of each OCR file, and the method are in the source record. Rights: Public domain (US)
The Supplement is not a Project Gutenberg text. It is Bibliotheca Catholica’s transcription of public-domain scans at the Internet Archive; obvious OCR errors were corrected, and passages marked “OCR uncertain” could not be confidently corrected, so check them against the linked scan page.
Objection 1: It seems that those who are not priests cannot excommunicate. Because excommunication is an act of the keys, as stated in iv. Sent. D. 18. But those who are not priests have not the keys. Therefore they cannot excommunicate.
Obj. 2: Further, more is required for excommunication than for absolution in the tribunal of Penance. But one who is not a priest cannot absolve in the tribunal of Penance. Neither therefore can he excommunicate.
On the contrary, Archdeacons, legates and bishops-elect excommunicate, and yet sometimes they are not priests. Therefore not only priests can excommunicate.
I answer that, Priests alone are competent to dispense the sacraments wherein grace is given: wherefore they alone can loose and bind in the tribunal of Penance. On the other hand, excommunication regards grace, not directly but consequently, in so far as it deprives a man of the Church's prayers, by which he is disposed for grace or preserved therein. Consequently even those who are not priests, provided they have jurisdiction in a contentious court, can excommunicate.
Reply Obj. 1: Though they have not the key of Orders, they have the key of jurisdiction.
Reply Obj. 2: These two are related to one another as something exceeding and something exceeded,* and consequently one of them may be within the competency of someone while the other is not.
[*Cf. A. I, ad. 2, Q. XXIV., A. i, ad. 1.]
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Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.